1. Field of the Invention
The present invention generally relates to a method and apparatus for diagnosing tomographic images acquired by an X-ray CT (computerized tomography) apparatus, or an MRI (magnetic resonance imaging) apparatus. More specifically, the present invention is directed to a tomographic image diagnosing method and an apparatus capable of setting a region of interest on the tomographic image with the recognition on positions of reference substances imaged in the tomographic image.
2. Description of the Related Art
As a conventional method for measuring an amount of bone mineral contained in an object under medical examination, e.g., a patient by way of an X-ray CT apparatus, the following comparison method has been known in the art. That is, both the object under medical examination and a so-called "phantom", i.e., an article having a plurality of reference substances therein are simultaneously imaged, and thereafter a comparison is made between an averaged CT value of the bone of the patient and a CT value (i.e., an X-ray absorption value) of each of the reference substance. In FIG. 1, there is an example for utilizing such a phantom in conjunction with the X-ray CT apparatus. In this figure, an object 2 under examination is mounted on a couch plate 12 above a couch 30 of the X-ray CT apparatus 1. A plate-like curved phantom 3 is interposed between this object 2 and the couch plate 12.
FIG. 2 illustrates a shape of this phantom 3. FIG. 2A is a plan view of the phantom 3, and FIG. 2B is a cross-sectional view, taken along the line I--I' and viewed along an arrow direction. As apparent from FIG. 2B, a width "H" of the phantom 3 is substantially equal to another width "H" (see FIG. 1) of the couch plate 12. One major surface "M" of the phantom 3 is placed on the couch plate 12, whereas the other major surface "N" thereof receives the object 2 under examination thereon. A plurality of reference substances "13a" to "13e", so-called "rods", are provided within the phantom 3. These reference substances "13a" to "13e" contain different amounts of a known, constant substance (for example, CaCO.sub.3) similar to the bone mineral. A base portion 15 in which no rod 13 is present is made of an urethane resin. Both the diameters of the respective rods and positions thereof are previously known. Both such a phantom 3 and the object 2 under medical examination are imaged by the X-ray CT apparatus 1 so as to obtain a CT (computerized tomographic) image as shown in FIG. 3. As previously explained, the plural rods 13a to 13e have been embeded within the phantom 3 on the couch plate 12, and the object 2 under examination is being positioned on the phantom 3. As shown in FIG. 3, regions of interest (referred to as "ROIs") "Ra" to "Rf" are successively set on the CT image by the manual operation of the operator with respect to the positions of the rods "13a" to "13e" and the bone 31 of the object 2 under medical examination. Since the respective pixels in the resultant image obtained by the X-ray CT apparatus 1 have the CT values inherently corresponding thereto, the amount of the bone mineral of the object 2 under examination can be predicted by comparing these CT values with the averaged CT values of the pixels of the respective ROIs "Ra" to "Rf". However, according to the above-described conventional ROI setting method, since ROIs "Ra" to "Rf" must be manually set by the operator while observing the CT image, the very cumbersome operation is necessarily unavoidable.
As another conventional ROI setting method with utilizing the positions of the rods contained in the phantom, another phantom 3' with a marker is employed as represented in FIG. 4. This phantom 3' contains a plurality of rods 13a' to 13e' similar to those of FIG. 2, and also two markers 32 and 33. These markers 32 and 33 have known very higher (or lower) CT values than the CT values of the organs inside the object 2 under examinations. FIG. 4A is a plan view of the phantom 3' and FIG. 4B is a cross-sectional view thereof, taken along a line J--J' and viewed along another arrow direction. It should be noted that the mutual positional relationships between the markers 32, 33 and rods 13a' to 13c' are previously known. In this conventional method, both the object 2 under examination and the phantom 3' with the markers 32 and 33 are simultaneously imaged by the X-ray CT apparatus 1, whereby a CT image as shown in FIG. 5 is acquired. Then, as the CT values of these markers 32 and 33 are already known, and there is no pixel having the CT value similar to those of the markers 32, 33, a measurement is carried out for the CT values of the respective pixels within the CT image, whereby the positions of these markers 32 and 33 of the phantom 3 can be readily recognized. Thus, as previously described, since the mutual positional relationships between the markers 32, 33 and the rods 13a' to 13e' are previously known, the positions of the respective rods 13a' to 13e' can be readily recognized. As a result, regions of interest (ROIs) can be set for the positions of the respective rods. However, according to the second conventional method, it is not possible to use a normal phantom, i.e., a phantom having no marker. The manufacture cost of such a specific phantom having markers becomes extremely high, as compared with the normal phantom without any marker.
Moreover, the similar disadvantages to those of the above-described X-ray CT apparatus are caused in another phantom containing the rods which have different water contents when this phantom is imaged in an MRI apparatus.
As previously described, in the conventional method where the positions of the reference substances contained in the phantom are recognized so as to set the regions of interest, the positions of ROIs must be manually set by an operator in case of using the normal phantom without markers. Moreover, the specific phantom with markers is required so as to set ROIs. In particular, when employing such a specific phantom, there are great possibilities that an artifact may occur from the marker, which will give an adverse influence to the image information required for the diagnosis.
The present invention has been made in an attempt to solve the above-described problems of the conventional ROI setting methods, and therefore has an object to provide such a tomographic image diagnostic method and also a tomographic image diagnostic apparatus capable of automatically recognizing positions of reference substances and also automatically setting a region of interest by employing the normal phantom having no marker without any cumbersome operation by an operator.